The present invention relates generally to radio-frequency (RF) antennas, and in particular to antennas used on unmanned air vehicles (UAVs). UAVs are increasingly used for surveillance, as communication repeater stations, or as targets in military applications. Scale model UAVs (model airplanes) are flown for recreation. In every application of a UAV, it is critical to maintain reliable RF communication between the vehicle and a ground station, which may be used for control of the vehicle or to transmit and receive communication signals.
Blade antennas are often used on aircraft, especially high-speed aircraft, for their convenient aerodynamic shape and generally omnidirectional radiation pattern, at least in azimuth. However, as the aircraft changes attitude, such as by turning or banking, the radiation pattern of the antenna is rotated with respect to the ground station, resulting in fluctuations in antenna gain. These fluctuations affect the link margin, which is generally defined as the amount by which a received signal exceeds a predetermined lower limit for desired message quality. Ultimately, the fluctuations adversely affect the range of the communication link. In addition, if the RF signals are vertically or horizontally polarized, the polarization angle of the antenna will vary with respect to the fixed polarization of an antenna at the ground station. These effects are for the most part tolerated in manned aircraft applications but have a more significant effect in UAV applications, in which it may be desirable to provide increased range and reliability of communication with ground stations.
Accordingly, there is a need for an antenna for UAVs that will provide better gain than blade antennas and yet be largely unaffected by changes in aircraft attitude. The present invention satisfies this need.
The present invention resides in the use of roll and pitch error signals from an unmanned air vehicle (UAV) to isolate an antenna mount from rolling and pitching movement of the vehicle, such that the antenna radiation pattern and polarization direction will not be affected by these movements of the vehicle. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, of which the following is a brief description.